Wave length modulation circuit



1944- G. L. USSELMAN WAVE LENGTH MODULATION CIRCUIT Filed Dec. 12, 1941 3 Sheets-Sheet 1 INVENTOR LI AQTII'ORNEY Feb.w29, 1944. e. USSELMAN 2,342,708

WAVE LENGTH MODULATION CIRCUIT Filed Dec. 12, 1941 a Sheets-Shget 2 INVENTOR Gear L ydefizm ATII'ORNEY Feb. 29, 1944. e. 1.. USSELMAN 2,342,708

WAVE LENGTH MODULATION CIRCUIT Filed Dec. 12, 1941 3 Sheets-Sheet 3 1 l'l-l'l-l- M T '0" @052 T REIT M M 5585 e5 \veg6 .INVENTOR fieoggeL. @fiebnaro AWW ATTORNEY Patented F eb. 29, 1944 T ()FFICIL WAVE LENGTH MODULATION CIRCUIT George L. Usselman, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application December 12, 1941, Serial No. 422,637

6 Claims.

This application concerns wave length modulation systems. .In particular, this application concerns a wave length modulation system comprising an electron discharge device with electrodes connected in a crystal-controlled oscillation generating circuit and other electrodes connected with an output circuit electronically coupled to the oscillation generating circuit with a reactance tube modulator comprising two electron discharge systems in reactive circuits coupled to the output circuit of the discharge tube wherein the oscillations are generated and also connected to a source of potentials representa tive of the signals it is desired to modulate the generated oscillations with. The modulator of the present invention is of the nature of the modulator disclosed in Goldstine application Serial No. 371,075, filed December 21, 1940, now U. S. Patent #2,306,052, dated December 22, 1942.

In describing my invention in detail, reference will be made to the attached drawings, wherein:

Fig, 1 is a circuit diagram illustrating the essential features of a wave length modulator arranged in accordance with my invention;

Figs. 2 and 3 are modificationsof the arrangement of Fig. 1; and

Fig. 4 is a simplified showing of a part of the circuits of Figs. 1 to 3 and is used with the vector diagram of Fig. 5 to explain the operation of the circuits of the prior figures.

Referring to Fig. 1, tube VI has its first and second grids GI and G2. connected with a piezoelectric crystal X to form an oscillation generating circuit, the frequency of the oscillation of which is dependent nearly entirely on the dimensions of the crystal X. The electrode GI is connected to the cathode K and to ground-byaresistance 2, while the electrode G2 is connected to a positive point on a source of direct current potential by way of a resistor 4. The source not shown has its negative terminal grounded and is shunted by a bypass condenser BC. The connections as described connects the crystal between the screen'grid G2, which serves as an anode, and the control grid GI with the cathode K connected to the control grid GI by resistance 2 to form an oscillation generator.

The anode A of tube VI is connected with a tank circuit comprising a variable condenser C3 and an inductance LI connected in parallel. A point on the inductance LI is connected to a source of direct current potential which may be the source mentioned above or a separate source.

This lead is filtered by a bypass and blocking condenser BC connected to ground. The carrier wave generated in VI and modulated as to phase in a manner described hereinafter is supplied to utilization means by means of inductance L2 coupled to LI Oscillations are produced in the circuits connected with electrodes GI and G2 0! tube VI of a frequency determined in large part by crystal X. These oscillations are supplied to the tank circuit LI-C3 connected with the anode -A sub.- stantially by the electron stream only of the discharge device VI. The tank circuit and the anode A are shielded from the oscillation generating circuits by a grounded electrode G3 arranged between the electrodes A and G2 and connected directly toground.

The modulator tubes V2 and V3 have their anodes III and I2 connected to spaced pointson the inductance LI of the tank circuit, for example, at the ends thereof. A movable tap on the inductance LI is connected by a two-stage phase shifting circuit comprising condenser C, resistance R0, condenser CI, and resistance RI to the cathodes of tubes V2 and V3 by way of ground. The grid electrodes I I and I6 are connected to a point on the phase shifting circuit C, R0, CI,

RI respectively, to excite the said grid electrodes by voltages of substantially like phase but of a phase displaced about ninety degrees with respect to the phase at the anode I0, and at its point of connection to inductance LI. Since the anodes I0 and I2 are connected to opposite ends of inductance LI, the phase of the excitation on 1 the grids I4 and I6 is also displaced ninety degrees with respect to the voltage on the anode I2. These tubes are fed so that the grids to the tubes are excited by voltages substantially ninety degrees out of phase with respect to the voltage on the respective anodes of the tubes. In one of the tubes, the grid voltage leads the "plate voltage by ninety degrees and in the other tube the grid voltage lags the plate vo1tage by-ninety degrees. The grid voltage is fed fromra'com mon point on one side of the plate circuit tofboth grids and since the two plates are one hundred and eighty degrees out or phase, this makes the grid voltages appear in their proper relationship to make one tube appear as capacitive and the other tube inductive reactance. As the reactances of the tubes are varied by means of the audio input, one tube will increase inductive reactance and the other will decrease capacitive reactance,

and vice versa. I

A feature of the present invention is the use of the novel two-stage phase shifter 0, R0, CI, RI, which permits a full ninety degree relation between the voltages on the grids and anodes of the respective tubes to be obtained. In order to facilitate understanding of the invention and in particular of the two-stage phase shifter, the same is shown separately in Fig. 4. In Fig. 4 el represents the voltage fed from Ll across C and ground while e5 represents the voltage between grids H and I8 and ground or cathode potential.

The two-stage phase shifter network consists of two stages of L connected condenser and resistance combination arranged as shown in the figures of this inventon of which Fig. 4 is a diagrammatic sketch to be used for explanation. In Fig. 4, el represents the excitation voltage obtained from the tank coil Ll of the invention.

This voltage enters the phase shifter network and comes out changed in phase as e5 in Fig. 4.

Now, in this invention, it is important that this excitation voltage of the modulator grids I4 and I8 should have a ninety degree phase shift as compared with the anode voltages in order that no amplitude modulation be produced coincident with phase modulation.

The ninety degree phase shift can easily be produced in the two-stage phase shifter, but a full ninety degrees phase shift cannot be obtained from a one stage phase shifter. This canbe seen by referring to the vector diagram Fig. 5 which represents the voltage and current between the input voltage el and the output voltage e5 is ninety degrees. This is easily obtained by making XC'=R for each element, that is XC=XCI =R0=R l It should be noted that in this invention the ninety degrees phase relation between the grids of the reactance tubes (modulator tubes) and the anodes not only make these tubes function much better as reactance modulators but it also balances out substantially all of the amplitude modulation leaving only the phase modulated carrier.

The novel arrangement here includingthe twostage phase shifter condensers is easily tuned.

In Fig. 1, condensers C and Cl can be mechanically coupled together such as a split stator condenser and adiustedby thesame handle. In

fact, condensers C, Cl and (33 can be coupled,

together'and controlled from one handle, in which'case condenser C3 is insulated from C and Cl.' Thisarrangement makes it very convenient for changing frequencies when different frequency crystals are switched in the oscillator circuit.

Thus, we see that tube-V2 may be considered as supplying a current to the tank circuit including inductance Ll, which is leading with respect to the current therein while the tube V3 may be considered as supplying a plate current' to the tank circuit Ll, which is lagging with respect to the, current therein so that the said a tank circuit is in effect looking in to a simulated capacity provided by tube V2 and a. simulated inductance provided by tube V3. Opposed variation of the tube conductances as a consequent raises and lowers the tank circuit reactance to thereby change the phase and/or frequency of the current therein. In order to have this modulator balanced, it is desirable that the phase displacement of the voltages on the anodes and grids of the respective tubes V2 and V3 is substantially ninety degrees and this phase relation is readily obtained by my improved two-stage phase shifting network.

Modulation is accomplished by applying modulating potentials from any source to the screen grid electrodes 20 and 22 in phase opposition. Increasing the potential on the grid 20 increases the current through the tube V2. Increasing the current through the tube V2 decreases the capacitive reactance provided by the said tube. This is equivalent to adding capacity to the tank circuit. Since the modulation is in phase opposition at this time the current through the other tube, say V3, is decreased, thereby increasing the inductive reactance provided by tube V3. This has the effect of adding inductance to the tank circuit. The added capacity and increased inductance shunt the tank winding LI and alters the tune thereof slightly. Both the added shunt capacity and the higher shunt inductance decrease the resonant frequency of the circuit. Increasing the potential of grid 22 and decreasing the potential on the grid 20 has the opposite effect. Thus, the tubes are controlled to vary the reactance of the tank circuit in accordance with the modulating potentials. This modulates the phase of the oscillatory energy in the tank circuit and the phase modulated oscillatory energy may be utilized directly from the tank circuit through inductance L2 coupled therewith or.

may be used after amplification and multiplication to the extent required in frequency multipliers and amplifiers. 1

The modification of Fig. 2 is similar in many respects to the modification in Fig. 1. In Fig. 2, the oscillatory circuit crystal is of the three-electrode type, one electrode of which is connected to the cathode, the other two electrodes being connected substantially directly to the grids GI and G2 so that this oscillator is of the Hartley type.

Inv Fig. 1,. screen grid modulation of the reactance tubes V2v and V3 is used whereas the phase shifting circuit of Fig. 2 is arranged to permit grid modulationof the reactance tubes V2 and V3. In Fig. 2, the resistor RI of the phase shifting network is replaced by the two resistors R2 and R3 which also serve as grid biasing resistances and means for feeding the modulated potentials from source 3 to the grids I4 and II. In this arrangement, condenser Cl is connected by coupling condensers CC to the high potential ends of resistors R2 and R3 which in, parallel perform to some extent at. least the function of resistor RI in the prior Fig. 1. This arrangement as will be seen, like the prior ar-q rangement, provides a balanced modulator which is desirable in arrangements of this type to cancel out amplitude modulation produced during the wave length modulation process. Negative po-' The arrangement of Fig, 3 is in many respects similar to the arrangement of Fig. 2. In Fig. 3, however, the ends of the tank circuit C3-L2 are a balanced substantially completed with respect to' ground, that is, with respect to the ground point on inductance L2 by supplying between one end of the tank circuit and ground a variable balance ing condenser B. This permits me to obtain complete symmetry of this tank circuit wherein the phase modulation takes place, with respect to ground. Although I have shown the balancing condenser B as being connected to the lower end of tank circuit L2C3, it will be understood that this showing is schematic and in practice it may be connected to that end having the least coupling to ground.

This phase modulator lends itself readily to be adapted to the transmission of telegraphy signals represented by direct current impulses of changing polarity or changing amplitude. This is accomplished by connecting the output impedance of an absorber modulator tube V4 in shunt to a resistance 24 supplying anode current to the osQllator tube by way of tank circuit inductance L2. When the impedance of tube V4 is low, a

' drain is placed on the oscillator anode supply circuit, thereby loweringfthe potential on the oscillator and as a consequence decreasing the amplitude of the generated oscillations. Th s keyer tube V4 may in a similar manner be used to key the plate circuit of the additional stage including V3, which has its grid 32 connected by coupling condenser CC and lead 36 to a point on the tank circuit (II-L2 to'ampl fy and/or multiply or otherwise relay the oscillations generated in the tank circuit.

Although I do not hereby limit my phase modulator system as illustrated in Fig. 3 to particular circuit values, the following circuit values were found to operate well in a system of this nature: resistance R0, 2500 ohms; res stances R2 and R3. 5000 ohms; resistor 22, 10,000 ohms; resistor 24, 10,000 ohms; resistor 26, 30,000 ohms; the potentiometer P being across -10 volts with respect to ground; phase shifting condensers C and Cl, to 80 uuf; grid condensers CC, .001 microfarad. The blocking and bypassing condensers BC are not critical; tubes VI and V2 were 6AC7 or GAG! while tube V4 was a type 2A3; V3 was a type 802.

What is claimed is:

1. In apparatus of the class described, a circuit comprising an inductance wherein wave energy flows, a pair of electron discharge tubes each having an anode, a cathode, and a control grid, a connection between a point intermediate the terminals of said inductance and the cathodes of said tubes, a connection between one terminal of said inductance and the anode of one of said tubes, a connection between the other terminal of said inductance and the anode of the other of said tubes, said last two named connections setting up on the anodes of said tubes voltages of the frequency of the wave energy flowing in said inductance and of opposed phase, a multiple stage phase shifter connecting a point on said inductance intermediate one terminal thereof and the point thereon connected to said cathodes to the control grids of both of said tubes, and connections for modulating the gain of said tubes different ally in accordance with control potentials.

2. In apparatus of the class described, a circuit comprising an inductance wherein wave energy, the wave length of which is to be modulated flows, a pair of electron discharge tubes each having an anode, a cathode, and a control grid, a connection between a point intermediate the terminals of said inductance and the cathodes of said tubes, a connection between one terminal of said inductance and the anode of one of said tubes, a connection between the other terminal of said inductance and the anode of the other of said tubes, said last two named connections setting up on the anodes of said tubes voltages of the frequency of the wave energy flowing in said inductance and of opposed phase, two phase shifting capacities in series connecting a point on said inductance intermediate one terminal thereof and the point thereon connected to said cathodes to the control grids of both of said tubes, a phase shifting resistance connecting the junction point of said capacities to the cathodes of said tubes, a second phase shifting resistance connecting the grids of said tubes to the cathodes of said tubes, and connections for modulating the impedances of said tubes differentially in accordance with signals.

3. In apparatus of the class described, a circuit comprising an inductance wherein wave energy, the wave length of which is to be modulated flows, a pair of electron discharge tubes ergy flowing in said inductance of opposed phase,

a multiple stage phase shifter connecting first and second points intermediate theterminals of said inductance to the control grids and cathodes respectively of said tubes, said multiple stage shifter comprising two stages with series reactance and shunt impedance in each stage, and connections for modulating the impedances of said tubes differentially in accordance with signals.

4. In a phase modulation system, a source of wave energy comprising an electron discharge device having electrodes coupled in a wave energy generating circuit including a frequency stabilizing crystal and an output circuit including a tuned inductance, a pair of electron discharge modulator tubes each having an anode, a cathode, and a control grid, means tying the cathodes of said tubes together and to a point intermediate the terminals of said inductance, a connection between the anode'of one of said tubes and one terminal of said inductance, a connection between the anode of the other tube and the other terminal of said inductance, said last two named connections setting up on said anodes phase opposed voltages of the generated frequency, a multiple stage phase shifter comprising series reactances and shunt impedances connecting a point on said inductance intermediate one terminal thereof and the point thereon to which the cathodes are connected to the control electrodes of said tubes to set up thereon like phase voltages displaced substantially with respect to the voltages set up on the anodes of the respective tubes, and means for differentially modulating the impedances of the tubes in accordance with signals.

ities.

6. Apparatus as recited in claim 4, wherein said tuned inductance is tuned by a variable capacity to i'orm'a parallel circuit tuned to the frequency or the generated wave energy and wherein said series reactances in the multiple stage phase shifter are variable, and a common control for said variable capacity and the variable reactances oi the phase shifter.

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